Evolution process and driving force analysis of natural wetlands in Xiliao River Basin based on SWAT model

Abstract: Natural wetland is one of the most productive ecosystems, providing with the wide arrays of ecological services to the local communities in particular and global communities in general. The Xiliao River Basin in the eastern edge of agro-pastoral ecotone in northern China, has become an important agricultural and animal husbandry production base, with obviously regional transition and vulnerability of ecosystem. It is necessary to accurate assesse the driving forces of wetland ecosystem evolution, thereby to understanding the long-term evolution of wetland ecosystem, and the contribution rate of climate and human factors to wetland change. In this study, taking the Xiliao River Basin as a research area, a quantitative analysis was made to investigate the evolution and hydrologic attribution of wetlands using the soil and water assessment tool (SWAT Model), Geo-detector, landscape conversion methods. The digital elevation model (DEM), land use, meteorological observation, and vegetation index data were firstly utilized to define the geographical region in the Xiliao River Basin, and then to explore the lake and marsh wetland areas, landscape conversion characteristics, change characteristics of marsh wetland vegetation, and river runoff depth, finally to quantify the types of natural wetland for a long time series and the transformation process. An emphasis was placed on the impact factors of climate or land use on wetland change and their interaction, as well as the contribution rate of driving factors to wetland change. The results showed that the average areas of lake and swamp were 607.14 km2, and 2 657.00 km2, respectively, both of which showed a decreasing trend. The average depth of rive runoff was 48.46 ± 36.82 mm, indicating no obvious change trend. Most types of land use had invaded the swamps and lakes, with the largest area occupied by cultivated lands. Since 1980, there was a dry-wet-dry transformation process occurred in the Xiliao River Basin, except for the wet periods in 1995-2000. From 2000-2015, the mean NDVI and the average annual GPP were 0.30 and 257.73 g/(m2·a) in the growing season of swamp wetlands, both of which showed an extremely significant upward trends and regional differences. There was a highly significant linear positive correlation between the changes of lake area, the runoff depth, precipitation and evapotranspiration, a highly significant conic relationship between the change of swamp area and the above two factors, and a highly significant linear positive correlation between NDVI, GPP, and accumulated temperature in the growing season. The influence of natural factors on wetland changes was greater than that of human factors. The precipitation was the main control factor of wetland changes in the Xiliao River Basin. A single factor driving effect showed that the precipitation was the main controlling factor of wetland change in Xiliao River Basin, where the order of driving capacity of precipitation and evapotranspiration on wetland change from large to small was the marsh area, river runoff, and lake area. A synergistic effect of precipitation and evapotranspiration on lake, swamp, and runoff depth was higher than 70%. This finding can offer the possible mechanisms to control the changes of wetland ecosystem in the Xiliao River Basin.